TABLE 4-2 Data from Table 3-8 in This Report on All Relevant Ductile Iron Pipe with Polyethylene Encasement and Cathodic Protection in Corrosive Soils

Abbreviated Project Name

Total Project Age, ti (years)

Average Age (years)

Length, li (miles)

ti × li

Failures

Failure Rate (failures/mile/year)

WEB

24

12

150

3,600

0

0

Southwest

25

20.5

75.8

1,895

1

0.000528

Mid-Dakota

12

9

49.6

595.2

0

0

Montrose

27

27

27

729

0

0

Hanna

20

20

7

140

0

0

Santa Margarita

23

23

7.5

172.5

0

0

Vacaville

24

24

3

72

0

0

Akron

7

7

0.25

1.75

1

0.571429

Trinidad

29

27.5

10

290

0

0

California City

25

16.5

1.5

37.5

1

0.026667

Denver

24

24

9

216

0

0

Sheridan

16

15

12

192

0

0

TOTALS

 

 

352.65

7,940.95

 

 

SOURCE: In Chapter 3 of this report, see Table 3-8, “Partial List of Cathodically Protected Polyethylene-Encased Ductile Iron Pipelines.”

year. This threshold value is only slightly lower than that provided by Reclamation (Threshold 1).

Probability Analysis for Threshold Value

The thresholds estimated in the previous subsections can be used to calculate the probability of having no failures over a 50-year period. Also, the data can be used to estimate the probability of having one or more failures. Assuming that the annual probability of failure per mile remains constant over time10 and that each year is statistically independent, the probability of having zero failures over a 50-year period for a given mile of pipe can be determined as follows:

Using the threshold values determined in the previous subsections and Equation 4-3, the probability of having no failures for a given mile of pipe over a 50-year period for each threshold value is shown in Table 4-3, and the trend in the probability with threshold is linear, as shown in Figure 4-2. The probability of

10

The failure rate generally increases over time. However, the failure rate here is for pipe having an average age near 50 years. Therefore, calculating the probability of failure using this rate will be conservative (i.e., it should overpredict the number of failures in 50 years).



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement